This invention relates to a method and apparatus for controlling power controllers in general and more particularly to a method and apparatus for controlling single or multiphase a.c. power controllers by using the phase-angle of semiconductor valves.
In order to automatically control the power supplied to an electrical load, e.g., a motor in an alternating current network, a.c. power controllers which are controlled by the phase-angle of semiconductor valves are used. The varying operating conditions that might occur require that the power supplied to the motor be automatically controlled in order to protect the network and the motor from unnecessary load transients. For example, such variations might result from the variable loading of the motor when it is being started or stopped. This is particularly true in the operation of an alternating-current motor. In order to automatically control the power supplied to the motor, it is necessary to determine the phase-angle between the voltage and the current.
A device for controlling an alternating current motor is disclosed in British Patent 2 084 359. The purpose of this device is to improve an unfavorable power factor, which may be caused, for example, by the underloading of the motor. For this purpose, controllable semiconductor devices, in particular a triac (a bidirectional triode thyristor) or an antiparallel thyristor circuit arrangement, are provided for each phase of the motor. By means of phase-angle control, these devices improve the power factor by using a method whereby the phase difference between the current and the voltage is detected in a control device for each phase. This phase difference is reduced by increasing the triggering angle, which is the time interval between the current zero crossing and the triggering point, for that particular phase.
This known device uses the time at which the current is zero as a reference time for determining the triggering point. The time of this current zero crossing is detected by measuring the voltage applied across the triac. This voltage is fed to a comparator, whose output states correspond to the states of this triac. The current zero crossing then corresponds to the waveform edge of the output signal of the comparator. From this comparator, with the aid of a monostable circuit, a series of pulses is produced for generating a ramp voltage which is synchronized with the voltage zero crossing of the supply voltage. The sampled value of the ramp voltage is subtracted from a reference voltage that is obtained externally via a potentiometer and transmitted to the inverting input of a differential amplifier. The output voltage of this differential amplifier is applied together with the ramp voltage to an additional comparator, which generates a primary triggering signal by means of a subsequent triggering pulse generator when the ramp voltage exceeds the output voltage of the differential amplifier. As a result of this circuit arrangement, the phase shift between the motor current and the motor voltage, and thus the power factor, is stabilized at a value which is given by the reference voltage adjusted by the potentiometer. Instead of determining the phase-angle by the analog method of using a ramp voltage synchronized with the system voltage, a digital measurement of the phase-angle that uses a counter is also known. The counter is started at the zero crossing of the voltage and stopped at the zero crossing of the current. The contents of the counter correspond to the phase-angle and can be used to control the power factor of the motor. The triggering angle is thereby varied until the actual phase-angle as measured equals the desired phase-angle (U.S. Pat. No. 4,361,792).
It has been shown that the measured phase-angle fluctuates from period to period due to disturbances which are superimposed on both the voltage and the current. These fluctuations of the measured phase-angle are then propagated, in the case of the known devices, to the triggering angle and lead to an unstable control response. In the automatic control of motors, for example, this results in the motor running in an irregular and unstable manner.
In view of the prior art, there is a need for a method and a device for controlling single or multiphase a.c. power controllers in which the fluctuations that occur from period to period in the measurement of the phase-angle do not lead to an unstable control response.